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    <h3 class="subtitle">14_eval函数</h3>
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        <h1 class="title">14_eval函数</h1>
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            <span>三月 29, 2022</span>
            
  <ul class="post-tags-list" itemprop="keywords"><li class="post-tags-list-item"><a class="post-tags-list-link" href="/tags/python%E5%AD%A6%E4%B9%A0/" rel="tag">python学习</a></li></ul>


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            <h1 id="eval-函数"><a href="#eval-函数" class="headerlink" title="eval 函数"></a><code>eval</code> 函数</h1><p><code>eval()</code> 函数十分强大 —— <strong>将字符串</strong> 当成 <strong>有效的表达式</strong> 来求值 并 <strong>返回计算结果</strong></p>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br><span class="line">4</span><br><span class="line">5</span><br><span class="line">6</span><br><span class="line">7</span><br><span class="line">8</span><br><span class="line">9</span><br><span class="line">10</span><br><span class="line">11</span><br><span class="line">12</span><br><span class="line">13</span><br><span class="line">14</span><br><span class="line">15</span><br></pre></td><td class="code"><pre><span class="line"><span class="comment"># 基本的数学计算</span></span><br><span class="line">In [<span class="number">1</span>]: <span class="built_in">eval</span>(<span class="string">&quot;1 + 1&quot;</span>)</span><br><span class="line">Out[<span class="number">1</span>]: <span class="number">2</span></span><br><span class="line"></span><br><span class="line"><span class="comment"># 字符串重复</span></span><br><span class="line">In [<span class="number">2</span>]: <span class="built_in">eval</span>(<span class="string">&quot;&#x27;*&#x27; * 10&quot;</span>)</span><br><span class="line">Out[<span class="number">2</span>]: <span class="string">&#x27;**********&#x27;</span></span><br><span class="line"></span><br><span class="line"><span class="comment"># 将字符串转换成列表</span></span><br><span class="line">In [<span class="number">3</span>]: <span class="built_in">type</span>(<span class="built_in">eval</span>(<span class="string">&quot;[1, 2, 3, 4, 5]&quot;</span>))</span><br><span class="line">Out[<span class="number">3</span>]: <span class="built_in">list</span></span><br><span class="line"></span><br><span class="line"><span class="comment"># 将字符串转换成字典</span></span><br><span class="line">In [<span class="number">4</span>]: <span class="built_in">type</span>(<span class="built_in">eval</span>(<span class="string">&quot;&#123;&#x27;name&#x27;: &#x27;xiaoming&#x27;, &#x27;age&#x27;: 18&#125;&quot;</span>))</span><br><span class="line">Out[<span class="number">4</span>]: <span class="built_in">dict</span></span><br></pre></td></tr></table></figure>

<h2 id="案例-计算器"><a href="#案例-计算器" class="headerlink" title="案例 - 计算器"></a>案例 - 计算器</h2><p><strong>需求</strong></p>
<ol>
<li>提示用户输入一个 <strong>加减乘除混合运算</strong></li>
<li>返回计算结果</li>
</ol>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line">input_str = <span class="built_in">input</span>(<span class="string">&quot;请输入一个算术题：&quot;</span>)</span><br><span class="line"></span><br><span class="line"><span class="built_in">print</span>(<span class="built_in">eval</span>(input_str))</span><br></pre></td></tr></table></figure>

<h2 id="不要滥用-eval"><a href="#不要滥用-eval" class="headerlink" title="不要滥用 eval"></a>不要滥用 <code>eval</code></h2><blockquote>
<p>在开发时千万不要使用 <code>eval</code> 直接转换 <code>input</code> 的结果</p>
</blockquote>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br></pre></td><td class="code"><pre><span class="line"><span class="built_in">__import__</span>(<span class="string">&#x27;os&#x27;</span>).system(<span class="string">&#x27;ls&#x27;</span>)</span><br></pre></td></tr></table></figure>

<ul>
<li>等价代码</li>
</ul>
<figure class="highlight python"><table><tr><td class="gutter"><pre><span class="line">1</span><br><span class="line">2</span><br><span class="line">3</span><br></pre></td><td class="code"><pre><span class="line"><span class="keyword">import</span> os</span><br><span class="line"></span><br><span class="line">os.system(<span class="string">&quot;终端命令&quot;</span>)</span><br></pre></td></tr></table></figure>

<ul>
<li>执行成功，返回 0</li>
<li>执行失败，返回错误信息</li>
</ul>

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